Processor Programming (Continued)
3.3.2.2 Configuration Registers
Each data transfer through I/O Port 23h must be preceded
by a register index selection through I/O Port 22h; other-
wise, subsequent I/O Port 23h operations are directed off-
chip and produce external I/O cycles.
The Configuration Registers listed in Table 3-9 are CPU
registers and are selected by register index numbers. The
registers are accessed through I/O memory locations 22h
and 23h. Registers are selected for access by writing an
index number to I/O Port 22h using an OUT instruction
prior to transferring data through I/O Port 23h. This opera-
tion must be atomic. The CLI instruction must be executed
prior to accessing any of these registers.
If MAPEN, bit 4 of CCR3 (Index C3h[4]) = 0, external I/O
cycles occur if the register index number is outside the
range C0h-CFh, FEh, and FFh. The MAPEN bit should
remain 0 during normal operation to allow system regis-
ters located at I/O Port 22h to be accessed (see Table 3-
11 on page 52).
Table 3-9. Configuration Register Summary
Access Default
Reference
Index
Type
Name
Controlled By*
Value
(Bit Formats)
C1h
C2h
C3h
E8h
EBh
20h
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
RO
CCR1 — Configuration Control 1
CCR2 — Configuration Control 2
CCR3 — Configuration Control 3
CCR4 — Configuration Control 4
CCR7 — Configuration Control 7
PCR — Performance Control
SMHR0 — SMM Header Address 0
SMHR1 — SMM Header Address 1
SMHR2 — SMM Header Address 2
SMHR3 — SMM Header Address 3
GCR — Graphics Control Register
VGACTL — VGA Control Register
VGAM0 — VGA Mask Register
SMAR0 — SMM Address 0
SMI_LOCK
--
00h
00h
00h
85h
00h
07h
xxh
xxh
xxh
xxh
00h
00h
00h
00h
00h
00h
4xh
xxh
Table 3-11 on page 52
Table 3-11 on page 52
Table 3-11 on page 52
Table 3-11 on page 53
Table 3-11 on page 53
Table 3-11 on page 53
Table 3-11 on page 54
Table 3-11 on page 54
Table 3-11 on page 54
Table 3-11 on page 54
Table 4-1 on page 97
Table 4-37 on page 163
Table 4-37 on page 163
Table 3-11 on page 54
Table 3-11 on page 54
Table 3-11 on page 54
Table 3-11 on page 54
Table 3-11 on page 54
SMI_LOCK
MAPEN
--
MAPEN
MAPEN
MAPEN
MAPEN
MAPEN
MAPEN
--
B0h
B1h
B2h
B3h
B8h
B9h
BAh-BDh
CDh
CEh
CFh
FEh
FFh
--
SMI_LOCK
SMI_LOCK
SMI_LOCK
--
SMAR1 — SMM Address 1
SMAR2 — SMM Address 2
DIR0 — Device ID 0
RO
DIR1 — Device ID 1
--
Note: *MAPEN = Index C3h[4] (CCR3) and SMI_LOCK = Index C3h[0] (CCR3).
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